Injection molding is a process by which material is melted and then injected into a mold cavity. Once the melted plastic is in the mold, it cools to a shape that reflects the cavity. The resulting form usually is a finished part, needing no other work. Many details, such as bosses, ribs, and screw threads can be formed during the one-step injection molding process. An injection molding machine has two basic components: an injection unit to melt and transfer the plastic into the mold, and a clamp unit to hold the mold shut against injection pressures, and for parts removal.

Injection molding processing conditions


Molding 90 rpm

Fast Injection


Injection 3-5 sec

Cooling 5-10 sec


800-1200 kg/cm2

600-1000 kg/cm2

Back pressure
5-10 kg/cm2


Back section

Center section

Front section


Mold 30-50

Injection Processing Requirements with High Transparency PP

Mold requirements The entire molding surface needs chromate treatment or at minimum glass polishing. If the molding surface is rough, there will be numerous unregulated tiny apertures that will influence light refraction, destroying transparency.
Water cooling should be arranged to surround the mold and be as evenly distributed as possible.
Injection method, position, and dimensions should be chosen to benefit fast-cycle forming.
Injection requirements Wall thickness: 1.2mm for standard, exceeding1.3mm and beyond, haze gradually increases.
Corner position: Use a large arc transition, to avoid shrinking and stress buildup.
Shape: Streamlined formed design aids in high speed shaping processing.
Molding processing Practically, resin temperature should be controlled around 210-230, when resin temp. is 240 and higher it will increase more. The mold temp. should be kept at around 50.
Use a high injection speed, adding Nitrogen gas will assist in even more ideal injection force.
Injection and maintained pressure should not be excessive, as to avoid producing deformed products.
Due to its transparent nature, transparent PP quickly crystallizes and its solidifying temp. is high. It can be 20-30 cooler in the same amount of cooling time as ordinary PP.
Note: In practice, the processing suggestions above are very important neglecting any of these conditions will reduce transparency, and other normal characteristics that should appear will be compromised. Transparent random copolymer's haze is normally slightly lower than transparent homopolymer, and definition is better, although temp. at which it changes shape tends to be lower (80-100), and is unsuitable for use with long periods of high temperature, as it will easily change shape. Transparent homopolymers threshold is higher (135-140).

Suggested Clamping Pressures for Injection Molding

ABS 0.45 to 0.65 T/cm2
LCP 0.75 to 0.8 T/cm2
PA 0.65 to 0.75 T/cm2
PBTP 0.65 to 0.75 T/cm2
PC 0.5 to 0.8 T/cm2
PE 0.15 to 0.55 T/cm2
PETP 0.65 to 0.75 T/cm2
PMMA 0.45 to 0.75 T/cm2
POM 0.85 to 1 T/cm2
PP 0.15 to 0.55 T/cm2
PPS 0.3 to 0.6 T/cm2
PS 0.3 to 0.5 T/cm2


Extrusion processing is defined as converting plastic granules into a continuous uniform melt and forcing the through a die which yields a desired shape. This melted material must then be cooled back to it solid state as it is held in the desired shape so an end-product can be realized. End products include custom profiles (auto trim, house siding), sheet (vacuum-formed products like cups, plates, and fridge liners), pipe, tubing, fibers, film (bags, coverings, and laminates), coatings, and blow-molded products (bottles and gas tanks). The melt must be shaped and cooled by product sizing and cooling equipment to its solid phase
while forming a product that falls within given size tolerances. The methods to create the end products from a melt are varied, depending on the shapes involved. Examples are sheet and blown film.

Processing Conditions

Processing conditions

Melt Index


Roller glossiness

The two rollers that contact the melted sheet must be highly polished.

Melt temperature

Homopolymer 235-245

Random copolymer 225-235

Roller temperature

Control homopolymer surface temp. at < 110

Control random copolymer surface at < 95

Scrap material for reuse

Cannot use material containing other residues.


After extruded the sheet is cooled and sized on highly polished and liquid-cooled rollers.

Conditions For Extruded Sheet Remolding

Processing conditions

Mold glossiness

General glossiness

Drift material

Material with low heat conduction and high gloss

Controlled temperature

Homopolymer 154-158

Random copolymer (3% ethlyene) 138-141

Mold temperature

Hot mold cavity causes heat shrinkage

Blown film

Blown film is normally extruded vertically in a tubular shape that cools, is collapsed, and wound up as a thin film. It is then either slit and wound into a thin sheet, or is converted directly into bags. The cooling and orientation of the melt as it travels upward from the die is done by high-speed air whose flow is directed by an air ring. Typically PP is blown down and water cooled while and PE is blown up and cooled by air.


Laminating a product with film is used for a number of reasons, such as decorating or printing a design onto a sheet, to protect a surface design against abrasion or water. Laminating is used in products such as flooring, furniture, and packing materials. A clear tough film when laminated with a printed or decorated film can withstand the effects of abrasion and exposure.


Thermoforming is the process of taking thermoplastic sheet and, using heat and pressure (or vacuum), to re-shape the sheet into useful products. 

Blow molding

Blow molding is a process for the production of hollow components. The most widely known blow molded objects are bottles, jars, jugs, cans, and containers of all kinds for the food, beverage, cosmetic, medical, pharmaceutical, and home products industries. Among other blow molded items are balls, bellows, and toys. For the automotive industry, fuel tanks, car bumpers, seat backs, center consoles, and armrest and headrest skins are blow molded. Below we will mention two kinds of blow molding--extrusion and injection


Extrusion blow molding makes up three-quarters of all blow molded products. It is a five step process that, in a nutshell, makes a product by forcing material through an orifice or die, and blowing that material against the cooled walls of a mold cavity.


Injection blow molding is a two-stage process for producing completely finished plastic containers. Plastic is injection molded into a preform cavity in the first step. The neck of the container is molded as well as the shape of the container. In the second step, air is blown into the preform forms it to the shape of the mold.


In this process, the machinery involved injection moulds a preform, which is then transferred within the machine to another station where it is blown and then ejected from the machine. This type of machinery usually requires large runs to justify the very large expense for the injection moulds to create the preform and then the blow moulds to finish the blowing of the container. This process is used for extremely high volume (multi-million) runs of items such as wide mouth peanut butter jars, narrow mouth water bottles, liquor bottles etc. 

Processing Parameters

Plastic resin manufacturing involves a process of melting resin pellets, controlling its flow, and then shaping and cooling a  inished product into a given shape. Below some of the general concepts in the manufacturing process are explained, and at the end of this section material testing methods are explained.

Temperature and Elasticity

Plastics chemical properties and flexibility have a close inseparable relationship, with the biggest influence being that of temperature. Every kind of materials' properties are influenced by varying temperature dissimilarities, but in plastics the dissimilarities of various types of resin relative to temperature difference is far greater.  With PP alone, chemical make up of the basic material can vary and differences in temperature can cause the same basic material to have different molecular weight (even reacting with different melt flow rates), thus requiring different temperatures in processing. Granted, different machine mold types also have different temperature requirements.

The elasticity of plastics correspond to levels of temperature, however the points discussed hereare only general concepts. A clearer explanation of plastic characteristics requires different testing methods to gather experimental data for differential comparisons. Ultimately however, data methods are limited because there is no way for an completely realistic phenomenon to occur.Here we will attempt to depict resin's changes under the influence of temperature.

When resin is in low temperature, molecules cannot freely move. Under these circumstances an appearance like that of brittle and easily breakable glass emerges. With temperature exceeding this degree, the range of molecular movement will increase alongside temperature increase. This phenomenon of molecular movement is generally called the Brownian movement. Naturally,
molecular movement makes plastics more pliable, so less external force is necessary to be applied for forming, and plastics are less easily breakable.

Polypropylene products have different requirements depending on how they are influenced by atmospheric temperature at different altitudes. When atmospheric temperature is higher, the range of molecular movement is relatively larger. If external force is applied incorrectly, it is difficult to restore the plastic to its original shape. Therefore temperature for heat molding and the softening point are two items for testing. When temperature is even higher, molecules can freely move, called the melting point, and can again be molded and cooled into their final shape. However, if the temperature is higher than standard, the material will burn and become charcoal-like, called the flashpoint. These are only a few concepts concerning temperature.


One of the first stages involved in processing is gradually melting the resin pellets to a fluid state, adjusted accordingly for different resin types. A higher temperature brings about a faster resin flow and can increase production efficiency, however it may not necessarily guarantee improved results, and a suitable equilibrium must be obtained. In production it is best to let the resin flow smoothly and freely to the mold head, to avoid insufficient or flow-back phenomenon from occurring, meaning the resin flow is being emitted faster than the rate of production. Ultimately a faster flow of resin indicates increased MFR. With MFR distribution in a more extreme condition, unstable processing conditions are also likely to increase. However, because the final products made with PP usually do not require highly precise measurements, this is typically not a big concern.


A big part of PP processing relies on the circulation that is propelled in the screw shaft, so itsdesign is very important. The caliber size influences production capacity, and its compression ratio size influences thrust value, as well as production capacity and product outcome. This includes many kinds of material (pigment, additives and agents) blending results. In the shaft, resin
becomes fluid mainly by the heater, however, some heat from the resin being rubbed together also contributes to fluidity.


Reshaping plastic often occurs through use of a mold. In injection molding, after resin is softened from the heat, material is injected into a mold. The mold has cavities that, when filled with the material, define the molded part. The material enters these cavities through passages cut into the mold, called runners.


Besides having channels for material to be injected into it, a mold is also designed with passages to circulate water to cool the hot plastic. After it cools into its final shape and the part is removed from the mold. In extrusion molding, there is water passages inside the rollers for cooling.


Resin experiences a problem of shrinkage, the reason being that the material expands while hot and retracts when cooled. Also during crystallization, internal stress is created. Generally speaking, the contraction and retraction is relatively easy to overcome by extending the cooling time during production and prolonging the applied pressure. Crystallized PP tends to have greater
shrinkage discrepancy than non-crystallizing PP. 


Header Image Credit: Jean-Pierre "Cjp24"
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